Somersetcounty, north-central New Jersey, U.S., bordered to the northeast by the Passaic River and to the east by Green Brook and the Raritan River. Its topography varies from lowlands in the east to a hilly piedmont region in the west. The principal waterways (in addition to the Green, Passaic, and Raritan) are the Millstone, Lamington, and Dead rivers. The primary forest species are oak and hickory. Parklands include Delaware and Raritan Canal Park and Washington Rock State Park.

The Dutch were early settlers in the region—originally inhabited by Delaware Indians—and were followed by a mix of English, Scottish, German, and French. The county was created in 1688 and named for Somerset, Eng. Somerville, the county seat, contains two state historic sites: the Old Dutch Parsonage (built 1751) and the Wallace House, General George Washington’s winter headquarters (1778–79) during the American Revolution. In 1783 Washington quartered in Rocky Hill at Rockingham—also a state historic site. Other towns include Bound Brook, South Bound Brook, Manville, and Raritan.

Health and business services, pharmaceutical manufacturing, and retail trade are the main industries. Area 305 square miles (789 square km). Pop. (2000) 297,490; (2007 est.2010) 323,552444.